package/iperf: bump version to 2.1.6

Changelog ([1]):

  - fix tos (-S) regression
  - fix --permit-key regression

[1] https://sourceforge.net/projects/iperf2/files/readme.txt/download



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>